This pull request fixes a panic that exists in both mempools. The panic occurs when the ABCI client misses a response from the ABCI application. This happen when the ABCI client drops the request as a result of a full client queue. The fix here was to loop through the ordered list of recheck-tx in the callback until one matches the currently observed recheck request.

This is, perhaps, the trival final piece of #7075 that I've been
working on.

There's more work to be done: 
- push more of the setup into the pacakges themselves
- move channel-based sending/filtering out of the 
- simplify the buffering throuhgout the p2p stack.

A fourth #7075 component patch to simplify the channel creation interface

This is another small sliver of #7075, with the intention of removing
the legacy shim layer related to channel registration.

Few things that had been annoying.

Addresses one of the concerns with #7041.

Provides a mechanism (via the RPC interface) to delete a single transaction, described by its hash, from the mempool. The method returns an error if the transaction cannot be found. Once the transaction is removed it remains in the cache and cannot be resubmitted until the cache is cleared or it expires from the cache.

The code in the Tendermint repository makes heavy use of import aliasing.
This is made necessary by our extensive reuse of common base package names, and
by repetition of similar names across different subdirectories.

Unfortunately we have not been very consistent about which packages we alias in
various circumstances, and the aliases we use vary. In the spirit of the advice
in the style guide and https://github.com/golang/go/wiki/CodeReviewComments#imports,
his change makes an effort to clean up and normalize import aliasing.

This change makes no API or behavioral changes. It is a pure cleanup intended
o help make the code more readable to developers (including myself) trying to
understand what is being imported where.

Only unexported names have been modified, and the changes were generated and
applied mechanically with gofmt -r and comby, respecting the lexical and
syntactic rules of Go.  Even so, I did not fix every inconsistency. Where the
changes would be too disruptive, I left it alone.

The principles I followed in this cleanup are:

- Remove aliases that restate the package name.
- Remove aliases where the base package name is unambiguous.
- Move overly-terse abbreviations from the import to the usage site.
- Fix lexical issues (remove underscores, remove capitalization).
- Fix import groupings to more closely match the style guide.
- Group blank (side-effecting) imports and ensure they are commented.
- Add aliases to multiple imports with the same base package name.

This is a little change just to make things more consistent ahead of
the 0.35 release.

This changes adds a failing test for issue #6660. It achieves this by adding a transaction, starting the `broadcastTxRoutine` in a goroutine and then adding another transaction to the mempool. The `broadcastTxRoutine` can receive the second inserted transaction before `insertTx` returns. In that case, `broadcastTxRoutine` will derefence a nil pointer when referencing the `gossipEl` and panic.

This changes adds an `MempoolError` field to the `ResponseCheckTx`. This will allow clients to understand that their transaction was rejected from the mempool despite passing the ABCI check. 

This change also updates the code to make use of early returns to prevent highly nested code blocks. Namely, it returns when the type assertion fails at the beginning of the method, instead of wrapping the entire method in a large if statement. This has a somewhat large effect on the diff as rendered by github.

addresses: #3546

closes #2207

Also added `Clear()` function if we want to clear whole elements in CList.

## Description

Move mempool errors to be public, this is used in handling abci error codes

Having smaller buffers in each reactor/channel will mean that there will be fewer stale messages.